This invention relates to methods of producing ultraviolet radiation absorbents for thermoplastic polymers. Films and various kinds of molded products such as boxes made of thermoplastic polymer materials undergo quality degradation such as discoloration and fading when exposed to ultraviolet radiation. Different kinds of ultraviolet radiation absorbents have therefore been in use for preventing such quality degradation for thermoplastic polymers as well as products made from them. The present invention relates to improvement in such ultraviolet radiation absorbents and methods of producing such improved absorbents.
Benzophenone compounds, benzotriazole compounds and salicylic acid compounds have been generally used as an ultraviolet radiation absorbent for thermoplastic polymers but it has been a problem that these absorbents are usually low in resistance against heat. In view of this problem, cyclic imino ester compounds have been proposed as ultraviolet radiation absorbents with improved heat resistance (U.S. Pat. No. 4,446,262, Japanese Patent Publications Tokko 62-5944 and 62-31027). These absorbents are themselves more resistant against heat but still have the problem that, when they are added or mixed to a thermoplastic polymer material such as polyethylene terephthalate or polycarbonate, they tend to adversely affect the original material characteristics of these thermoplastic polymers such as transparency. They also have the problem of sublimating and adversely affecting the workability and the environmental conditions during the mixing and molding processes when used for such thermoplastic polymer materials having high mixing and molding temperatures. There is also the problem that they do not store well although they may have to be stored for an extended period of time until they finally come to be used.